Conventional welding of precipitation hardened superalloys, (iron, nickel, and cobalt based), is particularly difficult because of solidification cracking and strain age cracking. Solidification cracking may occur during the welding process and strain age cracking may occur during a post weld heat treatment (PWHT) or exposure to high temperatures while in service. Arc welding, laser welding, electron beam welding, and similar processes melt the difficult-to-weld substrate. Cracking during resultant solidification is particularly difficult to avoid. Alternately, friction welding has been employed because friction welding only plasticizes the substrate without causing melting, thereby avoiding solidification cracking. However, friction welding has other disadvantages. For example, friction stir welding of superalloys is limited by short life of friction stir weld tool materials. A state of the art, high temperature poly crystalline boron nitride (PCBN) friction stir tool may be severely compromised after forming less than twelve inches of weld. Furthermore, high residual stresses are induced by the high forces involved in friction processing. These residual stresses can result in cracking (i.e. strain age cracking) during subsequent high temperature exposure.
Inertia friction welding (IFW) avoids solidification cracking and also avoids the problems of limited tool life associated with friction stir welding because in IFW the tool (i.e. rotating element) is incorporated into the friction process and is deposited onto and essentially forged onto the substrate. In the process a pin of material to be deposited is fixed in a chuck encompassed by a flywheel. The assembly is rotated to a high level of momentum. The motor is disengaged and the pin is forced under high pressure against a fixed substrate it is to be welded to. Material is plasticized by friction between the pin and substrate and is extruded out from the interface. An assembly where a pin has been inertia friction welded to a substrate can be seen in FIG. 1. Conventionally the welded assembly is subjected to a PWHT, after which cracks are often found in the weld. Such a crack can be seen in FIG. 2, across the interface where the pin material meets the substrate material. Consequently, there remains room in the art for a weld process and heat treatment that does not produce cracks in the weld of precipitation hardened superalloys.